Ability to see all books that you haven't finished reading (your bookmark exists, but you're not on the last page).

Ability to overwrite book titles or authors in case the metadata in the book isn't perfect.

LDAP.  So that someday this could be used in a large organization.

Ability to tag books with keywords and make them easily searchable.  Also, the ability to create "collections" based on those tags.

Ability to label books as a "favorite" for easy tracking of the ones you read the most or like the most (maybe even a rating system that keeps track of each user's rating and averages it).

Ability to make recommendations to other user's.

Swipe gestures for iOS Safari and Android browsers.

Keyword searches.  All my books are keyword search-able and I'd like to be able to search all the books in my collection for different keywords to determine which to go look at.